The present invention relates to a liquid crystal display driven by a lateral electric field drive mode and an electronic unit including the liquid crystal display.
In recent years, due to their low power consumption, space saving or the like, liquid crystal displays have become e mainstream displays. One of liquid crystal drive modes is a lateral electric field drive mode, such as an FFS (Fringe Field Switching) mode or an IPS (In-Plane Switching) mode. The lateral electric field drive mode is a drive mode for achieving display by forming an electric field in a direction parallel to a substrate and rotating liquid crystal molecules having a dipole moment in a plane parallel to the substrate. In particular, because the FFS mode has a simple electrode configuration in each pixel, the FFS mode is often used.
In the lateral electric field drive mode, a voltage is applied to each pixel electrode and a common electrode to form an electric field in a direction parallel to a substrate. When liquid crystal molecules are aligned in the direction of the electric field during voltage application, alignment deformation such as a so-called splay deformation or bend deformation occurs.
Liquid crystal molecules generally have shape asymmetry as illustrated in FIGS. 34A, 34B, 35A and 35B in addition to the dipole moment. When alignment deformation such as splay deformation or bend deformation occurs in a liquid crystal configured of such liquid crystal molecules, polarization may be induced. In other words, in a liquid crystal (a nematic medium), in the case where alignment deformation does not occur, as illustrated in FIG. 34A or FIG. 35A, polarization as a whole does not occur. On the other hand, when splay deformation occurs in a liquid crystal, as illustrated in FIG. 34B for example, polarization is induced, and when bend deformation occurs in a liquid crystal, as illustrated in FIG. 35B for example, polarization is induced. Such a phenomenon is known as a flexoelectric effect as described in J. S. Patel and Robert B. Meyer, “Flexoelectric electro-optics of a cholesteric liquid crystal”, Physical Review Letters, Volume 58, pp. 1538-1540, 1987, J. S. Patel and Sin-Doo Lee, “Fast linear electro-optic effect based on cholesteric liquid crystals”, Journal of Applied Physics, Volume 66, pp. 1879-1881, 1989, and the like.
In a liquid crystal display, to prevent degradation in a liquid crystal material, a so-called AC drive (or a frame reverse drive) is typically performed. In the AC drive, the polarity of a potential difference between the voltage of a pixel electrode and the voltage of a common electrode is reversed at regular intervals. In the case where a liquid crystal having the above-described flexoelectric effect is used for such a liquid crystal display, even if the polarity of the above-described potential difference is reversed in the AC drive, the polarity of polarization of the liquid crystal caused by the above-described flexoelectric effect is not simply reversed. As a result, light transmittance differs from pixel to pixel depending on the polarity of the potential difference. In particular, in the case where the AC drive is performed on such liquid crystal so as to reverse the polarity of a potential difference in each frame, light transmittance is different between a first frame (a positive frame), in which the voltage of the pixel electrode is larger than the voltage of the common electrode and a second frame (a negative frame), in which the voltage of the pixel electrode is smaller than the voltage of the common electrode. Accordingly, luminance of the liquid crystal display varies from frame to frame, and flickers occur on a screen to cause a decline in image quality.
A large number of methods of preventing the influence of a flexoelectric effect on image quality have been considered. For example, Japanese Patent No. 3668844 proposes a liquid crystal display configured by dividing each pixel into two regions in an IPS mode, and arranging a pixel electrode and a common electrode in one of the regions and a pixel electrode and a common electrode in the other region in interchanged positions so that electric field directions in the two regions are opposite to each other. Such a configuration prevents the occurrence of a difference in light transmittance in a pixel between a positive frame and a negative frame.
Moreover, a large number of methods for reducing the chances of causing a flexoelectric effect have been considered. For example, Japanese Unexamined Patent Application Publication No. 2009-167228 proposes a liquid crystal display using a liquid crystal having a less asymmetric molecular structure. Liquid crystal molecules are designed to have a structure in which the asymmetry is reduced in directions of an electron withdrawing group and an electron donating group.
In some liquid crystal displays, for example, as described in Japanese Unexamined Patent Application Publication Nos. 2002-131767, 2002-131780, H10-186407 and 2009-103925, a light-shielding layer is provided in a part of a pixel. The light-shielding layer is provided in an IPS mode to shield light in a part of a pixel where liquid crystal molecules are misaligned due to production of an unintended electric field between a pixel electrode and a pixel signal line. In other words, light in a part of a pixel where the alignment of liquid crystal molecules is not sufficiently controlled due to a pixel signal is shielded so as to prevent that part from affecting display. However, in the liquid crystal display disclosed in Japanese Patent No. 3668844, the electrode configuration in each pixel is complicated. In the liquid crystal display disclosed in Japanese Unexamined Patent Application Publication No. 2009-167228, because the molecular structure of the liquid crystal is complicated, material cost may increase, or the viscosity or birefringence of the liquid crystal may decrease. Further, in the case where various specifications are to be met, it is necessary to make a molecular design such as to obtain a molecular structure satisfying the specifications, and development cost may be expensive.
In Japanese Unexamined Patent Application Publication Nos. 2002-131767, 2002-131780, H10-186407 and 2009-103925, methods of preventing a decline in image quality caused by a pixel signal in an IPS mode liquid crystal display are disclosed, but a flexoelectric effect is not described therein. Therefore, a specific method of preventing a decline in image quality caused by a flexoelectric effect is not disclosed.
Therefore, it is desirable to provide a liquid crystal display and an electronic unit achieving high image quality with a simple configuration, without complicating an electrode configuration or a molecular structure of a liquid crystal.